Asymmetrical Diruthenium Complex Bridged by a Redox-Active Ligand

The asymmetrical dinuclear complex [(acac)₂Ru1­(μ-abpy)­Ru2­(Cym)­Cl]­PF₆ ([2]­PF₆), with acac^– = acetylacetonato = 2,4-pentanedionato, abpy = 2,2′-azobis­(pyridine), and Cym = p-cymene = 1-isopropyl-4-methylbenzene, has been obtained from the mononuclear precursors [Ru­(acac)₂(abpy)] and [Ru­(Cym)­Cl₂]₂. X-ray crystal structure analysis suggests the oxidation state formulation [(acac)₂Ru1^III(μ-abpy^•–)­Ru2^II(Cym)­Cl]⁺ for 2⁺, with antiferromagnetic coupling between one Ru^III center and the radical-anion bridging ligand (abpy^•–), based on the N–N distance of 1.352(3) Å. As appropriate references, the newly synthesized mononuclear [(abpy)­Ru^II(Cym)­Cl]­PF₆ ([1]­PF₆) with an unreduced NN double bond at d(NN) = 1.269(4) Å and the symmetrical dinuclear [(acac)₂Ru^2.5(μ-abpy^•–)­Ru^2.5(acac)₂] with d(NN) = 1.372(4) Å (rac isomer) support the above assignment for 2⁺ as an asymmetrical mixed-valent configuration bridged by a radical ligand. Reversible one-electron oxidation leads to a dication, 2²⁺, with largely metal-centered spin (EPR: g₁ = 2.207, g₂ = 2.155, and g₃ = 1.929), and a weak intervalence charge-transfer absorption at 1700 nm, as observed by spectroelectrochemistry. These results support a description of 2²⁺ as [(acac)₂Ru1^III(μ-abpy⁰)­Ru2^II(Cym)­Cl]²⁺. Density functional theory (DFT) calculations suggest that the first reduction of [2]­PF₆ also involves the bridging ligand, leading to [(acac)₂Ru1^III(μ-abpy^2–)­Ru2^II(Cym)­Cl] (2). Experimentally, the first reduction of 2⁺ is not fully reversible, with evidence for the loss of chloride to form [(acac)₂Ru1­(μ-abpy)­Ru2­(Cym)]⁺ (2a⁺; g₁ = 2.454, g₂ = 2.032, and g₃ = 1.947). Further reduction produces [(acac)₂Ru1^II(μ-abpy^2–)­Ru2^II(Cym)] (2a), which forms [(acac)₂Ru1^II(μ-abpy^2–)­Ru2^I(Cym)]⁻/[(acac)₂Ru^II(μ-abpy^•–)­Ru⁰(Cym)]⁻ (2a^–) in yet another one-electron step (g₁ = 2.052, g₂ = 2.008, and g₃ = 1.936). The major electronic transitions for each redox state have been assigned by time-dependent DFT calculations.